great idea! the time frame for somthing like this is probably a couple, or so, years out.
it always takes time for the appropriate tech to filter down to the end users, but that is
good. it gives us time.

my advice would be to form a small architecture team, 2 to 3 people at most, and agree
on the basic design. this team would establish the basic archtitecture "rules of the game"
and probably build the main world. other themed worlds could be farmed out to individuals
who sign up. this could be manageable, maybe, if these rules are in place. i would think
they would need to address standards for object persistency, security, model scale, etx.
i think there would probably need to be some consistent user interface items each world
would be expected to follow.

as someone who has seen many s/w projects fail or succeed, i would emphatically suggest
to keep it very, very simple at first. also, my experiences with web sites, simulations and
vr has told me that the artistic stuff is the hard part. we will need lots of help in that area.
unless you want something that looks like a bunch of s/w weenies did it.